Remove unused imports, do PEP8 and DeprecationWarning cleanups

mayan/apps/document_indexing/__init__.py

@@ -1,6 +1,6 @@
 from __future__ import absolute_import
 
-from django.db.models.signals import post_save, pre_delete, post_delete
+from django.db.models.signals import post_save, post_delete
 from django.dispatch import receiver
 from django.utils.translation import ugettext_lazy as _